In document copier machines of the electrophotographic type charged latent images are produced on a photoreceptive material and then developed through the application of a developer mix. Where the photoreceptive material is separate from the copy paper itself, a transfer of the developed image to the copy paper takes place with subsequent fusing of the developed image to the paper. A common type of developer mix currently in use in such machines is comprised of a carrier material, such as a magnetic bead, coated with a colored powdery substance called toner. It is the toner which is attracted to the charged, latent image to develop that image and it is the toner which is then transferred from the latent image to the copy paper (where the copy paper is separate from the photoreceptive material). Finally it is the toner which is then fused to the copy paper to produce the finished copy.
It is apparent from the procedure outlined above that toner is a supply item which must be periodically replenished in the developer mix since the toner is carried out of the machine on the copy paper as a reproduced image. It is also apparent that the concentration of toner particles in the developer mix is significant to good development of the latent image since too light a toner concentration will result in too light a developed image and too heavy a toner concentration will result in too dark a developed image.
Other variables which seriously affect copy quality include the image voltage of the photoconductor and the bias voltage on the developer. Many other variables factor into these basic quantities, for example, the quality of the original, the cleanliness of the optical system, and the condition of the photoconductor.
For a quantity control system that would attempt to accurately control toner concentration, image voltage, and other quality rendering factors, the control system itself must be designed to be as free from inherent error as possible. Broadly, this invention seeks to attain that general object. The most pertinent prior art relating to this problem known to the inventor is in the area of toner concentration control. That art includes U.S. Pat. Nos. 2,956,487 and 3,348,522. U.S. Pat. No. 2,956,487 provides a toner concentration control system where the reflectivity of the image to be reproduced is used as a measure of toner density. This system appears subject to difficulty since reflectivity readings will change dependent upon the quality of the original. U.S. Pat. No. 3,348,522 discloses a toner concentration control scheme in which a special test image is developed outside the image area used for reproducing document copies. In this latter patent separate reflectivity-sensing devices are used to simultaneously sense light reflected from a single light source, one sensing device to establish a voltage indicative of clear photoconductor outside the image area and the other to establish a voltage indicative of the test area which, as noted above, is also outside the image area. U.S. Pat. No. 3,348,523 is essentially similar to U.S. Pat. No. 3,348,522.
U.S. Pat. No. 3,926,338 discloses a circuit for use in a toner concentration control scheme. In this patent thermally insensitive photodetectors must be used since the large amount of heat generated during machine operation affects the accuracy of toner concentration control readings. Similarly, this patent says that a stable amplifying circuit, stable referring to temperature stability, must be used in order to avoid destruction of the validity of the sensed signal.
A better way has been discovered and is claimed in U.S. patent application Ser. No. 894,956; named above. Instead of producing a test area on a part of the photoconductor remote from the image area, it has been discovered that it is superior to provide a test cycle and place the test area within the image area itself. In that manner, the advantages of using a developed image are combined with the advantages of using the very same photoconductor that is used for document reproduction. It was found that on short runs the test cycle could be made to correspond to a run-out cycle after the last copy had been produced. However, during long, multi-copy runs, it may be necessary to skip an occasional copy in order to provide a test cycle. Test cycles may be kept relatively infrequent, once every 10 copies, for example, or even less frequent, since the use of the image rea as a test area produces significant advantages in accuracy. Some reasons for this include the fact that as photoconductor ages with use, there is a tendency for toner to build up on the image area; that the photoconductor surface characteristics change with use, thus affecting development; and that the photoconductor suffers electrostatic degradation with use. A result of these factors is that the image area itself becomes darkened as compared to the areas of the photoconductor which are not used for image impressions and the photoconductor does not charge as well as it does when fresh. When photoconductor charge is reduced, the voltage levels of a resultant latent image are changed as compared to new photoconductor. As a result, copies are produced which are too light. However, in the system described herein, where the toner concentration control test area or the image voltage test area are produced within the image area any results of toner filming, aging, use, etc., are present in the quality tests. Consequently, the absolute quantity of toner in the developer mix can be adjusted as the photoconductor changes and the value of the developer bias voltage can be changed to provide compensating factors for the effects of change. Such results are not possible unless the quality tests are taken within the image area. Even if the tests are taken within the image area, there is still no assurance that the results will be accurate unless the testing circuit is able to compare the resulting quantities to a meaningful reference and unless the quantities are devoid of circuit-induced non-linearities. This invention is directed toward a testing circuit, method and apparatus which does provide a meaningful reference and does produce a result which is relatively free of circuit-induced non-linearities, noise and changes in temperature.
The inventors herein have discovered that it is advantageous to view a cleaned, uncharged area of the photoconductor within the image area in order to provide a reference voltage. The prior art schemes outlined above used a reference voltage obtained from outside the image area and consequently not subject to the variables named above. Additionally, the inventors herein discovered that various elemental factors such as temperature as well as component non-linearities prevented accurate comparisons of reference voltage and sensed voltage unless the identical sensor is used for both measurements and unless it is excited to similar levels during both measurements. In this regard, the inventors herein discovered that the amount of light received for both sample and reference measurements by the sensor must be made equal (at the correct quality level) to avoid photodetector non-linearities and an ingenious circuit arrangement to provide this property was invented.
In the system described herein a reference voltage is allowed to vary from test to test by viewing a "bare" area of the photoconductor. The fact that the reference voltage is sensed each time a test is made by the same photodetector used to sense the developed image provides an extremely important advantage in that the variables associated with temperature, such as the effect of shifts in the magnitude of the dark current of the photodetector and shifts in the light output from the light source, are minimized. Other factors such as changes in the optical characteristics of the photoconductor due to oxidation and surface changes are also minimized. As a consequence of this dynamism the system becomes insensitive to temperature, becomes insensitive to variations in component qualities, and insensitive to other variables as noted. In the systems described in the prior art, few of these variables were ever compensated, most of them were not even considered.
Moreover, as taught and claimed in U.S. patent application Ser. No. 894,957; referenced above, by sensing the reference voltage during a test cycle from a bare photoconductor area that is used for the production of copies, additional quality-sensing capabilities are provided such as the sensing of an abnormally low reflectance photoconductor, i.e., a photoconductor on which toner buildup has produced a darkened condition or where the cleaning station or the erase means has malfunctioned such that an area of the photoconductor that should be clear is instead producing low reflectance.
Still another capability of the test apparatus is the means to partially check itself for proper functioning during periods when it is not in use. Therefore, when its use is needed, the machine is at least partially assured that it will receive correct indications of the measured qualities. This feature is taught and claimed in U.S. patent application Ser. No. 894,957.